Update: A huge thanks to GuanoLad on the Straight Dope message board for uncovering the installation's provenance: it's Ken Unsworth's Rapture, on exhibition at the Art Gallery of New South Wales.
From the AGNSW website:
Rapture both promises and denies a transcendent musical experience – a melodic stairway to heaven that never eventuates. We imagine striding up the keyboard stairs to a realm populated by harp-playing angels; instead at the top we get a muffled piano full of straw and inhabited by pesky mice. The only composition written on this instrument is that of the mice tripping down the stairs in playful and incoherent staccato.
Burnt music sheets at the top of the piano lend the work a melancholic air, hinting at the connection between music and emotional memory. Unsworth is interested in personal and collective symbolism, as well as the formal qualities of a sculpture that seems poised simultaneously to slide and levitate.
(Via the Sydney September 2009 set in thefourthcraw's Flickr photostream)